Electroless copper plating solutions with accelerated plating rates

ABSTRACT

THE RATE OF PLATING COPPER ONTO AN ACTIVE METALLIC SURFACE OF A SUBSTRATE FROM AN ELECTROLESS COPPER PLATING SOLUTION IS ENHANCED BY ADDING TO THE PLATING SOLUTION AN IONIC COMPOUND CONSISTING OF EITHER AN ACETATE, A NITRATE, AN OXALATE, A LACTATE, A CHLORIDE, A TARTRATE, A FORMATE, A PHTHALAE, A TUNGSTATE, A MOLYBDATE, A CHLORATE, A PERCHLORATE, A CITRATE, A MOLONATE, OR MIXTURES THEREOF.

United States Patent 3 720,525 ELECTROLESS COPPER PLATING SOLUTIONS WITH ACCELERATED PLATING RATES Nathan Feldstein, Kendall Park, and Joel Alan Weiner, Cranbury, N.J., assignors to RCA Corporation No Drawing. Filed Aug. 15, 1971, Ser. No. 172,300

Int. Cl. C23c 3/02 US. Cl. 106-1 4 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION This invention relates generally to electroless plating solutions, and more particularly to improved electroless copper plating solutions with accelerated plating rates.

The process of electroless plating is used in the manufacture of printed and hybrid circuits in the electronic industry. Generally, a relatively thin film of copper is electrolessly deposited upon a specially prepared surface of a dielectric substrate, and the deposited film is subsequently electroplated with copper to a desired thickness. Thus, the substrate may be electrolessly plated initially with a copper film of about 0.01 mil, for example, and then electroplated with copper to a thickness of between 1-2 mils. Since the rate of the electroless deposition of copper onto a substrate is relatively slow, it is advantageous to enhance the deposition rate, thereby increasing the rate of production and consequently reducing the cost per unit of construction.

It has been proposed to accelerate the rate of the electroless deposition of copper by increasing the temperature of the plating solution, but this procedure is not always desirable or practical because increased temperatures may either reduce the stability of some plating solutions and/or adversely affect certain materials that are temperature sensitive, such as photoresists, that may already be on the substrate.

The improved copper plating solutions of the present invention may be used at normal room temperatures and yet provide enhanced rates of deposition of copper in comparison with the electroless copper plating solutions of the prior art.

SUMMARY OF THE INVENTION Each of the improved electroless copper plating solutions with accelerated plating rates comprises the combination of an electroless copper plating solution with an accelerator that is a compound consisting of either an acetate, a nitrate, an oxalate, a lactate, a chloride, a tartrate, a formate, a phthalate, a tungstate, a molybdate, a chlorate, a perchlorate, a citrate, a malonate, or mixtures thereof. A preferred embodiment of an improved accelerated electroless plating solution comprises one wherein a cupric i-on complex is the reaction product of an ethyleneaminoacetic acid, preferably propylenediaminetetraacetic acid, and a cupric compound. The propylenediaminetetraacetic acid, (hereinafter referred to as PDTA), should contain no more than by weight, of nitrilotriacetic acid (hereinafter referred to as NTA),

3,720,525 Patented Mar. 13, 1973 DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention, the rate of deposition of copper from an electroless copper plating solution is enhanced by adding certain ionic accelerators to the plating solution. These ionic accelerators are relatively inexpensive salts that form an improved combination with electroless copper plating solutions to enhance the rate of deposition of copper therefrom without the necessity of increasing the operating temperature of the plating solution. Also, these accelerators are not consumed during the plating process.

The ionic accelerators for combination with electroless plating solutions to provide improved solutions are soluble salts of an acetate, a nitrate, an oxalate, a lactate, a chloride, a tartrate, a formate, a phthalate, a tungstate, a molybdate, a chlorate, a perchlorate, a citrate, a malonate, or mixtures thereof. While these ionic accelerators may have any cation, as for example, sodium, potassium, or ammonium cations, the sodium salts of the accelerators are preferred because they are usually the least expensive. These novel ionic accelerators when used in combination with prior-art electroless plating solutions markedly accelerate the plating rate. The improved electroless plating solutions may also contain wetting agents, bufiers, brighteners, and stabilizers, well known in the art.

Most electroless copper plating solutions are similar in their essential composition. They generally contain metallic cations as the source of metal to be deposited, a reducing agent, a chelating (or complexing) agent to prevent precipitation of any insoluble metallic salts, and pH regulators to maintain the solution between a pH of 10 and 14. A preferred electroless copper plating solution for combination with one or more of the accelerators of the present invention to enhance the rate of deposition of copper therefrom is as follows:

ELECTROLESS COPPER PLATING SOLUTION I 15.0 g./l. CuSO -5H O 53.0 m1./l. 40% active solution of PDTA (as Na, salt) 15.0 mL/l. 37% H CO 4.0 g./l. NaOH 4 10- g./l. NaCN 4.1 10- g./l. TMN (TergitoP a non-ionic surfactant of Union Carbide Co.-a trimethyl monylether of polyethylene glycol containing 6 moles of ethylene oxide) In the electroless copper plating Solution I, the copper sulfate is a source of metallic copper, providing cupric ions. The formaldehyde provides the reducing power for the bath. The PDTA is the complexing agent used to complex the copper ions to prevent the precipitation of copper as a hydroxide. The sodium hydroxide adjusts the pH of the solution to the proper alkalinity. The cyanide stabilizes the Solution; and the Tergitol is a Wetting agent that also functions, in combination with the cyanide, to brighten the deposited copper. The quantities of the ingredients of the Solution I, and other solutions described herein, are not critical and may vary within limits well known in the art. The PDTA used as the chelating agent in Solution I is generally not a pure compound because the pure product is relatively expensive. Commercial PDTA is usually sold with nitrilotriacetic acid (NTA) as a combined by-product, but the NTA should not be present in a quantity of more than 10%,

by weight, of the product. NTA also functions as a chelating agent.

PDTA is an ethyleneaminoacetic acid. Other ethyleneaminoacetic acids that may be substituted for the PDTA in Solution I in suitable amounts are ethylenediaminetetraacetic acid, diethylenetriaminepentaacetic acid, and 1,2-cyclohexylenediaminetetraacetic acid.

The novel ionic accelerators of the following Table I were combined with the electroless plating Solution I and the increase in the plating rate was noted:

TABLE I.IONIC ACSELERA'IORS IN SOLUTION OF Molar Mg. of Percent ity in copper increase Soluated plat- Ionic accelerator tion I out ing rate None (control) 8 N aCHKCOQ (sodium acetate) 1 13.0 62 NaNOa (sodium nitrate) 1. 12. 8 60 K2C204H2O (potassium oxalate) 15. 4 93 NaC H5Oa (sodium lactate)- 2 13. 5 69 NaCl (sodium chloride) 4 11.9 49 NaKCsHrOs (sodium potassium tartrate). 1 12.7 59 NaHCOz (sodium formate) 2 14. 4 79 NaKC HiO4 (sodium potassium phthalate 1 14. 7 84 Na1WO4 (sodium tungstate) 1 13. 0 6 2 N8zM0O (sodium molybdate) 1 12. 9 61 N 210103 (sodium chlorate) 1 12.8 60 NaClOr (sodium perchlorate)--. 1 12.1 51 Na3CsH501 (sodium citrate) 1 10. 7 34 N meal-I10 (sodium malonate) 1 10.0 25

In each of the examples of the combinations of the ionic accelerators and Solution I given in Table I, the Solution I was first saturated with nitrogen to remove any dissolved oxygen. Also, in each example, copper was allowed to plate out from the solution onto an active metallic surface of a substrate of a 2-inch square ceramic wafer at a temperature of 25 C. for a period of 10 minutes. The amount of an ionic accelerator that may be combined with an electroless plating solution is determined by the solubility of the accelerator in the solution. For practical purposes, the amount of ionic accelerator used should form at least a 0.1 molar solution with the electroless copper solution. Actually, the ionic accelerator may be added to the electroless plating solution until the solution is saturated. The percentage of increase in the rate of deposition of copper from the solution is substantially proportional to the amount of the ionic accelerator used. For example, successive applications of sodium acetate in concentrations on the order of 0.5, 1.0, and 1.5 moles to the electroless copper plating Solution I accelerated the plating rates 25%, 62%, and 93%, respectively.

Any clean active metallic surface that is not adversely affected by alkaline aqueous solutions can be plated with the copper plating Solution I merely by bringing the active metallic surface into contact with the plating solution, or by suitable pretreatment known in the art. 'Iypical active metallic surfaces that can be copper plated by the Solution I are, for example, clean surfaces of iron, cobalt, gold, silver, palladium, platinum, rhodium, copper, and alloys thereof.

Non-metallic surfaces can be copper plated by the Solution I but the non-metallic surfaces must first be provided with a catalytic metal to accept the copper plating. It is preferable to first roughen the non-metallic surface either by chemical or mechanical means, such as by chemical etching or by sand blasting. The nonmetallic surface is next treated with a sensitizing stannous chloride solution, followed by washing with water and then treating with an activating solution of a silver, gold, or palladium salt that will provide a catalytic film of the particular metal used. Thus, the non-metallic surface is provided with an active metallic surface on which the copper can be plated when brought into contact with an electroless copper plating solution.

ELECTRODES COPPER PLATING SOLUTION II 15.0 gr./l. CuSO '5H O 53.0 ml./l. PDTA Na, (40% solution) 4.0 gr./l. NaOH 15.0 ml./l. H CO (37% solution) 0.004 gr./l. NaCN TABLE II Weight 01 Percent Cu deposincrease in Ionic accelerator added ited (mg.) plating rate ELECTROLESS COPPER PLATING SOLUTION III 53.0 ml./l. PDTA Na, (40% solution) 4.0 gr./l. NaOH 15.0 ml./l. H CO (37% solution) 0.00014 gr./l. Tergitol TMN TABLE III Weight 01' Percent Cu deposincrease in Ionic accelerator added ited (mg.) plating rate ELECTROLESS COPPER PLATING SOLUTION IV 15.0 gr./l. CuSO -5H O 53.0 mL/l. PDTA Na (40% solution) 4.0 gr./l. NaOH 15.0 ml./l. HQCO (37% solution) TABLE IV Weight 01 Percent Cu deposincrease in Ionic accelerator added ited (mg.) plating rate ELECTROLESS COPPER PLATING SOLUTION V 7.5 gr./l. CuSO -5H O 15.0 gr./l. sodium salt of ethylenediaminetetraacetic acid (EDTA Na 20.0 gr./l. NaOH 40.0 ml./l. H CO (37% solution) 0.1 gr./1. NaON TABLE V Weight 01 Percent Cu deposincrease in Ionic accelerator added ited (mg.) plating rate None 10. 2 O 15. 0 47 14. 8 40 5 ELECTROLESS COPPER PLATING SOLUTION VI 15.0 gr./l. Cu(N -3H O 10.0 gr./l. NaHCO 20.0 gr./l. NaOH 100.0 ml./l. H CO (37% solution) TABLE VI Weight of Percent 011 deposincrease in Ionic accelerator added ited (mg.) plating rate None 16. 6 NaCHiCO 19. 3 16 KNO; 19. 4 17 ELECTROLESS COPPER PLATING SOLUTION VII 10.0 gr./l. NaHCO 20.0 gr./l. NaOH 100.0 ml./l. H CO (37% solution) 0.02 gr./l. NaON TABLE VII Weight of Percent Cu deposincrease in Ionic accelerator added ited (mg.) plating rate ELECTROLESS COPPER PLAT ING SOLUTION VIII 5.0 gr./l. CuSO -5H O 5.0 gr./l. NaOH 30.0 ml./l. H CO (37% solution) 25.0 gr./l. Na C H O -2H O The Solutions V, VI, and VIII, without any accelerating compounds, are described in detail in US. Patents 3,095,309, 2,874,072, and 3,259,559, respectively.

The electroless plating solutions described herein were premixed, but the reducing agent (formaldehyde) was separated from the oxidizing agent (copper salt) for storage purposes. This precaution was taken because of the thermodynamic metastability of electroless copper plating solutions. Where a cyanide was used, it was premixed with the formaldehyde and kept separated from the sodium hydroxide until needed to prevent the rapid deterioration of the solutions.

Several different substrate materials, such as laminated plastic boards commonly used in the manufacture of printed circuits, were also copper plated with good results. The Z-inch square alumina ceramic wafers (#5 16 Plain- American Lava Corporation), having a thickness of 0.025" and a surface roughness of 40 micro-inches, however, were used to obtain the data shown in Tables I-VIII because of their uniformity of surface roughness. It should be noted that the data in Tables I-VIII Would ordinarily vary with variations in the surface roughness of the substrates used.

The substrates of the experiments were spun dried and weighed before and after the copper deposition cycle. In

all of the experiments, the volume of each of the electroless plating solutions was maintained at 200 ml., and the copper plating was carried out without any external agitation. Also, in all of the experimental data recorded in Tables I-VIII, only a single component, the ionic accelerator salt, was varied while the other components were held constant. It has also been observed that in those electroless copper plating solutions from which the initial deposits of copper were dull in color, the addition of a cyanide and a non-ionic surfactant to the solutions increased the brightness of the copper deposited. Although the mechanism by which the ionic accelerator salts enhance the plating rate of electroless copper plating solution is not understood, the combination of these ionic accelerators with the electroless plating solutions provide improved solutions that markedly accelerate the plating rate of copper without substantially altering the quality of the copper deposited. When more than one of the ionic accelerators were combined with an electroless copper plating solution, the percentage of plating rate increase was at least as great as it would be if the ionic accelerator providing the greatest rate increase were added alone.

While the improved electroless copper plating solutions may be used satisfactorily at room temperature, their use is not limited thereto. The improved solutions also exhibit accelerated copper plating rates at elevated temperatures.

We claim:

ll. In an electroless copper plating solution which comprises a source of cupric ion, a reducing agent for cupric ion and a pH regulator, the improvement consisting of an additional ingredient which is a plating accelerator in an amount to provide a concentration between 0.1 molar and a saturated solution thereof, where said additional ingredient is a compound selected from the group consisting of sodium or potassium or ammonium nitrate, oxalate, lactate, chloride, formate, phthalate, tungstate, molybdate, chlorate, perchlorate, malonate, and mixtures thereof.

2. A solution according to claim 1 in which said source of cupric ion is a complex which is the reaction product resulting from mixing solutions of an ethyleneaminoacetic acid and a soluble cupric compound.

3. A solution according to claim 1 in which said source of cupric ion is a complex which is the reaction product resulting from mixing solutions of propylenediaminetetraacetic acid and a soluble cupric compound.

4. In an electroless copper plating solution which comprises a source of cupric ion, a reducing agent for cupric ion, a complexing agent for cupric ion and a pH regulator, the improvement consisting of an additional ingredient which is a plating accelerator in an amount to provide a concentration between 0.5 molar and a saturated solution thereof, where said additional ingredient is sodium, or potassium or ammonium acetate.

References Cited UNITED STATES PATENTS 3,310,430 3/1967 Schneble et a1 106-1 3,370,974 2/ 1968 Hepfer 106-1 3,377,174 4/1968 Torigai et al 106-1 3,515,563 6/1970 Hodoley et al 106-1 3,615,733 '10/1971 Shipley et a1 l06l 3,663,242 5/197-2 Gulla et al. 106-1 LORENZO B. HAYES, Primary Examiner US. Cl. X.R. 

